What does H7 actually mean?

The letter is the position of the tolerance zone and the number is the IT (International Tolerance) grade — the zone's width. "H" means a hole whose lower limit sits exactly at the nominal size; "7" is a precision grade common for machined holes. So a 25 H7 hole is 25 mm at its smallest, opening up by the IT7 amount.

Hole-basis vs shaft-basis — which do I use?

Hole-basis (vary the shaft against a fixed H hole) is the default, because holes are harder to make to size and standard reamers/drills produce H holes. Use shaft-basis only when the shaft is fixed by a standard (e.g. drawn bar or a bearing journal).

How do I get the exact numbers?

Look up the nominal-size row for the hole (H7…) and shaft (g6, p6…) in an ISO 286 limits-and-fits table; combine them for the clearance/interference range. This page tells you which fit to choose, not the per-size deviations.

Reference & Charts

Fits & Tolerances

The common ISO 286 preferred hole-basis fits — from loose running (H11/c11) to force fits (H7/u6) — with the fit type and what each is used for, plus how the hole-basis system and IT grades work.

Last verified: 2026-07-09

A fit is the clearance or interference between a hole and a shaft of the same nominal size, set by their tolerance zones. In the hole-basis system (the usual choice), the hole is the fixed reference — always an "H" zone with its lower limit at nominal — and the shaft letter/grade is varied to get the fit. Lower-case letters are shafts, upper-case are holes; the number is the IT grade (smaller = tighter).

Preferred hole-basis fits

Common ISO 286 hole-basis fits (hole / shaft)
Fit	Type	Typical use
H11/c11	Clearance — loose running	Wide clearance; non-critical, dirt/heat, big tolerances
H9/d9	Clearance — free running	Generous clearance; no accuracy needed, large temperature swings
H8/f7	Clearance — close running	Running fit at moderate speed/load with good lubrication
H7/g6	Clearance — sliding	Parts that slide/locate accurately with minimal clearance
H7/h6	Clearance — locational	Precise location, easy assembly by hand, no movement in service
H7/k6	Transition	Accurate location; small clearance or slight interference
H7/n6	Transition — tight	Accurate location with more interference; mallet/press assembly
H7/p6	Interference — light press	Rigid location; press fit, parts not meant to come apart
H7/s6	Interference — medium drive	Permanent/shrink-fit assemblies (e.g. bushings, collars)
H7/u6	Interference — force	Heavy interference for high loads; check hoop stress on the hub

Exact upper/lower deviations depend on the nominal size and are read from ISO 286 (or an equivalent limits-and-fits table). This chart selects the fit; look up the numbers for the specific diameter.

How to use it

Decide the relationship: should the parts move (clearance), locate precisely (transition), or stay fixed (interference)?
Pick the preferred fit from the table for that behaviour — H7/g6 and H7/h6 cover most precision locating; H7/p6 and up are press/shrink fits.
Look up the actual tolerance limits for your nominal diameter in an ISO 286 table, then put them on the drawing (as limits, or as the ISO callout like ⌀25 H7/g6).
For interference fits, verify assembly force and the stress in the outer part (hub) — heavier fits (s6/u6) can overstress thin hubs.

Frequently asked

What does H7 actually mean?: The letter is the position of the tolerance zone and the number is the IT (International Tolerance) grade — the zone's width. "H" means a hole whose lower limit sits exactly at the nominal size; "7" is a precision grade common for machined holes. So a 25 H7 hole is 25 mm at its smallest, opening up by the IT7 amount.
Hole-basis vs shaft-basis — which do I use?: Hole-basis (vary the shaft against a fixed H hole) is the default, because holes are harder to make to size and standard reamers/drills produce H holes. Use shaft-basis only when the shaft is fixed by a standard (e.g. drawn bar or a bearing journal).
How do I get the exact numbers?: Look up the nominal-size row for the hole (H7…) and shaft (g6, p6…) in an ISO 286 limits-and-fits table; combine them for the clearance/interference range. This page tells you which fit to choose, not the per-size deviations.

Sources

ISO 286-1 / ISO 286-2 — ISO code system for tolerances on linear sizes (limits & fits)
ANSI B4.2 — Preferred metric limits and fits